Conventionally, a cooling apparatus for a vehicle having a first coolant circuit, in which coolant circulates through an engine, and a second coolant circuit, in which the coolant circulates without passing through the engine, has been proposed. This arrangement allows coolant to flow independently in the respective first and second coolant circuits. Specifically, in the cooling apparatus, the first coolant circuit is used to cool the engine, and the second coolant circuit is employed to recover exhaust heat from the engine and heat the passenger compartment, as described in, for example, Patent Document 1.
FIG. 19 represents the configuration of the cooling apparatus described in Patent Document 1. The coolant in the first coolant circuit of the cooling apparatus is sent from a first water pump 52 and flows through the interior of an engine 50. The coolant then reaches a radiator 53 downstream from the engine 50, which radiates heat from the coolant. Afterwards, the coolant returns to the first water pump 52 via a thermostat 54. The thermostat 54, which is arranged in the first coolant circuit, operates in response to the temperature of the coolant flowing into the thermostat 54 to selectively prohibit and permit flow of the coolant through the radiator 53. The coolant circulating in the second coolant circuit is pumped out from a second water pump 55 and flows through a heater core 56, an exhaust heat recovery device 51, and a three-way valve 57 before returning to the second water pump 55. The heater core 56 heats air to be sent into the passenger compartment using the heat produced by the coolant. The exhaust heat recovery device 51 exchanges heat with exhaust gas from the engine 50 to recover the heat from the exhaust gas. The three-way valve 57 regulates the flow of the coolant. A coolant temperature sensor 60 is arranged in the second coolant circuit and detects the temperature of the coolant at a position downstream from the second water pump 55. The first coolant circuit and the second coolant circuit are connected to each other through a coolant passage 58 and a coolant passage 59. The coolant passage 58 connects the downstream side of the engine 50 to the three-way valve 57. The coolant passage 59 connects the downstream side of the exhaust heat recovery device 51 to the thermostat 54.
In this conventional cooling device for a vehicle, the thermostat 54 closes when the temperature of the coolant flowing into the thermostat 54 is low to block the coolant flow through the thermostat 54. The three-way valve 57 is controlled in correspondence with the temperature detected by the coolant temperature sensor 60. When the detected temperature is low, the three-way valve 57 connects the exhaust heat recovery device 51 to the second water pump 55. When the detected temperature is high, the three-way valve 57 connects the engine 50 to the second water pump 55. The first water pump 52 is controlled in correspondence with the temperature detected by the coolant temperature sensor 60 and stopped when the detected temperature is low.
In FIG. 20, the arrows represent the coolant flow at the time when the temperature of the coolant at the position downstream from the second water pump 55, which is detected by the coolant temperature sensor 60, is low. In this state, the thermostat 54 is closed and the three-way valve 57 operates to connect the exhaust heat recovery device 51 to the second water pump 55. This separates the first coolant circuit from the second coolant circuit. Further, in this state, the first water pump 52 is stopped and the second water pump 55 is operated solely. Accordingly, in the cooling apparatus of the vehicle, the coolant circulates only in the second coolant circuit. Specifically, the coolant flows from the second water pump 55 to the heater core 56 and the exhaust heat recovery device 51 and returns to the second water pump 55. On the other hand, the engine 50 retains coolant that is prevented from circulating. This causes a temperature rise in the coolant and thus promotes warm-up of the engine 50. If, in this state, the passenger compartment is heated, the coolant heated by the heat from the exhaust gas in the exhaust heat recovery device 51 is sent to the heater core 56. As a result, the air discharged into the passenger compartment is heated by the heat of the exhaust gas recovered by the exhaust heat recovery device 51.
In contrast, in FIG. 21, the arrows represent the coolant flow at the time when the coolant temperature at the position downstream from the second water pump 55, which is detected by the coolant temperature sensor 60, is high. In this state, the thermostat 54 is opened and the three-way valve 57 operates to connect the engine 50 to the second water pump 55. The first water pump 52 and the second water pump 55 are both in operation. As a result, in the cooling apparatus for a vehicle, a first circulation loop and a second circulation loop, as will be described below, are formed as two circulation loops for the coolant. The first circulation loop extends from the first water pump 52, proceeds through the interior of the engine 50, the radiator 53, and the thermostat 54, and returns to the first water pump 52. The second circulation loop branches from the first circulation loop after the coolant has passed through the engine 50. The second circulation loop extends through the second water pump 55, the heater core 56, and the exhaust heat recovery device 51 and remerges with the second circulation loop at the thermostat 54. At this stage, the coolant in the first coolant circuit is mixed with the coolant in the second coolant circuit. Accordingly, if the coolant in the second coolant circuit has been sufficiently heated by the heat from the exhaust gas in the exhaust heat recovery device 51 by the time when the coolant is mixed with the coolant in the first circulation loop, the coolant flowing into the engine 50 is heated through the coolant mixing, which promotes warm-up of the engine 50.